Project Summary We, and others have demonstrated that HIV-1 infects and preferentially depletes a majority of gastrointestinal (GI) CD4+ T cells within 2-3 weeks of infection. However, while GI mucosal T cells are well studied, there is very little information on the impact of HIV-1 on mucosal B cells. There are multiple reasons to study the mucosal B cell compartment in HIV. First, it is clear that B cells are critical to mucosal homeostasis through complex and wide ranging effects on the microbiome, the epithelial surface, and the resident innate and adaptive immune system. Second, dysfunction of systemic B cells is considered to be a hallmark of HIV-1 pathogenesis. And finally, that the intestines harbor the largest reservoir of B cells in the body. Therefore, we contend that studying mucosal B cells is central to a better understanding of the pathogenesis of HIV-1 infection in the mucosal and systemic compartments. In our preliminary studies we demonstrate a striking loss of Nave B cells from the GI tract of patients with acute HIV-1 infection (AHI) and multiple perturbations within GI B cell subsets during viremic, chronic HIV- 1 infection (CHI). These results support our hypothesis that HIV-1 results in major and perhaps irreversible alterations within the mucosal B cell compartment that are pivotal to the ensuing dysfunction of the mucosal immune system and contribute to the impaired integrity of the mucosal barrier. The proposed research will be carried out through a collaborative effort between three laboratories with distinct and complementary expertise uniquely suited to address the central hypothesis of this application. The PI of this application, Dr. Saurabh Mehandru is a trained gastroenterologist with more than 12 years of experience in the study of intestinal lymphocytes. Dr. Mehandru has made several key contributions to our understanding of the effect of HIV-1 infection on the GI immune system. Dr. Andrea Cerutti is world-renowned expert in B cell biology, mucosal immunology and studies of B cell dysfunction in HIV-1 infected individuals. Dr. Michel Nussenzweig, also the world's leading expert on B cells has pioneered the therapeutic use of innovative, broadly neutralizing antibodies (bNAbs) in HIV-1 infected patients. In addition, the research will be done in close collaboration with Dr. Judith Aberg, Head of Infectious Diseases at Mount Sinai and Dr. Uri Laserson who has an established track record in analyzing the clonal repertoire of B cells in HIV-1 infection. This proposal will allow us to comprehensively immunophenotype GI mucosal B cells, determine the impact of combination antiretroviral therapy (cART) on mucosal B cell reconstitution and correlate changes within mucosal B cells with CD4+ T cell depletion and mucosal barrier function defects (aim 1); map the clonotypic architecture of mucosal B cells before and after cART and bNAb therapy (aim 2); and study the effect of two bNAbs on the GI immune system and the level of HIV-1 within the GI tract (aim 3).